The invention relates to a process for producing a hafnium extraction scrub solvent for extracting (or "scrubbing") hafnium values from aqueous mixtures of zirconium and hafnium oxychloride. It is particularly concerned with the production of a thiocyanic acid-containing organic solvent for use in the start-up of a liquid-liquid extraction process.
The commercial production of zirconium metal from zircon sand involves the chlorination of the sand, which produces an intermediate process stream generally comprising a zirconium/hafnium oxychloride aqueous solution. The metal values may, if desired, then be separated in accordance with known practices. In one of these practices, the hafnium values are scrubbed from the mixed oxychloride-containing aqueous stream in a liquid-liquid process with a so-called "scrub"solvent comprising an originally barren organic solvent (usually methyl isobutyl ketone, also known as "MIBK") which has been made up to contain about 2-3 Molar of thiocyanic acid.
In a conventional start-up of a hafnium extraction system, scrub solution is made up by contacting an aqueous solution of dilute sulfuric acid containing thiocyanic acid with a barren organic solvent containing little, if any, thiocyanic acid, such as fresh or regenerated MIBK. As used in the context of the present disclosure, a "barren" organic solvent is one that has a thiocyanic acid concentration of less than about 1 Molar. Typically, even regenerated barren solvents contain substantially less thiocyanic acid. The aqueous solution preferably contains less than about 100 ppm hafnium ions, and more preferably less than about 50 ppm hafnium ions, so that hafnium-SCN complex ions do not consume the thiocyanic acid. An aqueous ammonium thiocyanate solution, typically having a pH of about 10, is added to the process to produce thiocyanic acid in the aqueous solution. The thiocyanic acid then gradually transfers to the organic solvent and builds up to the desired scrub solvent concentration of about 2 Molar or more. This system runs at the inherent pH of the aqueous stream. This conventional start up practice requires an extended period of time to achieve a scrub solvent concentration of 2 Molar or more.
Concentrated sulfuric acid was added in a conventional start up of a production facility with the expectation that an increase in the acidity of the dilute, thiocyanic acid-containing, sulfuric acid solution would more quickly drive the thiocyanic acid into the barren organic solvent. However, after three or four days into the start up, the thio ("SCN") concentration of the scrub solvent was found to be only 1.0-1.3 Molar. In addition, a large quantity of thio-contaminated sulfuric acid was produced, with consequent loss of the thio.
A process for starting up a hafnium extraction system in a shorter time with less waste generation and loss of feed materials is needed to more efficiently separate hafnium from zirconium.